Process for preparing aluminum base alloys

ABSTRACT

THE PRESENT DISCLOSURE RELATES TO A PROCESS FOR OBTAINING ALUMINUM BASE ALLOYS CONTAINING MAGNESIUM HAVING IMPROVED STRESS CORROSION RESISTANCE AND HIGH STRENGTH.

United States Patent 3,556,872 PROCESS FOR PREPARING ALUMINUM BASE ALLOYS George J. Jagaciak, Conyngham, Pa., assignor to Olin Corporation, a corporation of Virginia No Drawing. Division of applications Ser. No. 457,487, now Patent No. 3,346,370, Ser. No. 457,488, now Patent No. 3,346,371, Ser. No. 457,489, now Patent No. 3,346,372, Ser. No. 457,490 now Patent No. 3,346,373, Ser. No. 457,509, now Patent No. 3,346,374, Ser. No. 457,510, now Patent No. 3,346,375, Ser. No. 457,511, Ser. No. 457,515, now Patent No. 3,346,376, Ser. No. 457,516, now Patent No. 3,366,476, and Ser. No. 457,523, now Patent No. 3,346,377, all dated May 20, 1965. This application July 19, 1967, Ser. No.

Int. Cl. C22f 1/04 U.S. Cl. 148-115 11 Claims ABSTRACT OF THE DISCLOSURE The present disclosure relates to a process for obtaining aluminum base alloys containing magnesium having improved stress corrosion resistance and high strength.

The present application is a divisional of copending applications Ser. No. 457,487, filed May 20, 1965, now U.S. Pat. No. 3,346,370, Ser. No. 457,488, filed May 20, 1965, now U.S. Pat. No. 3,346,371, Ser. No. 457,489, filed May 20, 1965, now U.S. Pat. No. 3,346,372, Ser. No. 457,490, filed May 20, 1965, now U.S. Pat. No. 3,346,373, Ser. No. 457,509, filed May 20, 1965, now U.S. Pat. No. 3,346,374, Ser. No. 457,510, filed May 20, 1965, now U.S. Pat. No. 3,346,375, Ser. No. 457,511, filed May 20, 1965, now abandoned, Ser. No. 457,515, filed May 20, 1965, now U.S. Pat. No. 3,346,376, Ser. No. 457,516, filed May 20, 1965, now U.S. Pat. No. 3,366,476, and Ser. No. 457,523, filed May 20, 1965, now U.S. Pat. No. 3,346,377, all by George J. Jagaciak.

The present invention relates to new and improved aluminum base alloys containing magnesium. More particularly, the present invention resides in aluminum base alloys containing from 5.5 to magnesium and characterized by improved physical properties such as high strength and stress corrosion resistance.

The advantages to be derived from alloying magnesium with aluminum base alloys were recognized very early in the development of aluminum technology. Consequently, the aluminum-magnesium series of alloys is one of the oldest used commercially.

The development of inert gas shield are methods in welding in recent years has stimulated additional interest in sheet and plate of the stronger alloys in this series. In addition, the excellent physical properties of these alloys in welded structures is well recognized, such as the high yield strength obtainable without heat treatment, good weldability and good ductility.

Attempts have frequently been made to increase the magnesium content of the aluminum base alloys in wrought form up to 10%. These attempts, however, have not resulted in commercialization of aluminum base alloys containing more than 5.5% magnesium because of inherent problems of stress corrosion susceptibility of these alloys in the cold worked condition. Therefore, at the present time there are no satisfactory commercially available aluminum base alloys containing more than 5.5% mag nesium in cold worked tempers. Y

It is, therefore, highly desirable to develop such alloys due to the excellent physical properties which they promise, such as light weight, high strength levels equivalent to those of mild steel, excellent ductility and weldability. However, the inherent problems of stress corrosion susceptibility of these alloys in the cold worked tempers must be overcome. In other words, aluminum base alloys containing greater than 5.5% magnesium are generally not used at present commercially in strain hardened tempers because of their great susceptibility to stress corrosion cracking.

Accordingly, it is a principal object of the present invention to provide a process for obtaining new and improved aluminum base alloys containing greater than 5.5% magnesium.

It is a further object of the present invention to provide a process as aforesaid wherein the alloys are characterized by excellent physical characteristics, such as high yield strength, good weldability and good ductility.

It is a still further and particular object of the present invention to provide a process as aforesaid wherein the alloys overcome the great susceptibility of this type of alloy to stress corrosion cracking.

It is a further object of the present invention to provide a convenient and expeditious process for obtaining the aforesaid alloys.

Further objects and advantages of the present invention will appear hereinafter.

-It has been found that in accordance with the process of the present invention that the foregoing objects and advantages may be readily attained by:

(A) providing an aluminum base alloy consisting essentially of: from 5.5 to 10.0% magnesium; from 0.05 to 0.3% chromium; preferably a material selected from the group consisting of indium from 0.002 to 0.80%, gallium from 0.01 to 0.50%, cadmium from 0.03 to 0.50%, boron from 0.001 to 0.350%, thorium from 0.005 to 0.350%, misch metal from 0.005 to 0.30%, hafnium from 0.05 to 0.7%, tellurium from 0.005 to 0.30%, lithium from 0.01 to 0.80%, manganese from 0.05 to 1.0%, germanium from 0.01 to 0.55%, and cobalt from 0.10 to 0.80% plus copper from 0.10 to 0.60%, balance essentially aluminum;

(B) hot rolling said alloy at a temperature of from 450 to 950 F. to a gage of less than two inches; and

(C) cold rolling said alloy.

The alloys are preferably cold rolled to intermediate gage, although they may be cold rolled to final gage directly, if desired. The amount of cold reduction is limited by mill capability.

It has been found surprisingly that the process of the present invention achieves alloys which overcome the heretofore noted disadvantages of the art. Particularly surprising is the unusual stress corrosion resistance of the alloys of the present invention. For example, environmental stress corrosion tests were run in a rigorous atmosphere with the following results: various alloys of the present invention containing about 7% magnesium, about 0.15% chromium, and additives of the present invention, balance essentially aluminum, were subjected for a period in excess of one year with no stress corrosion failures, with the test still proceeding; whereas, substantially the same alloys without the additives of the present invention exhibited stress corrosion failure in 300 days; and substantially the same alloys without the additives of the present invention and without chromium exhibited stress corrosion failure after days of exposure.

Prior to the hot rolling step, it is preferred to provide W a heat treatment or homogenization step at from 850 to 975 F. for from to 30 hours and preferably to 16 hours.

Preferably the alloys are stabilized after cold rolling by holding at a temperature of from 200 to 450 F for at leastlS minutes and preferably 1 to 4 hours; however, the alloys may, if desired, be utilized in the cold rolled condition.

In the preferred embodiment the following additional process steps are performed after cold rolling but before stabilizing in the event that more cold rolling reduction is necessary or desired or if the material is required in the annealed temper: (D) annealing at a temperature of 500 to 1000 F., and preferably 650 to 950 F. for at least 5 minutes at least 60 minutes; and (E) cooling said alloys, preferably at a rate of 50 F. per hour or less, to room temperature. After the intermediate anneal, the alloys may again be cold rolled to the desired temper. This sequence of annealing, cooling, and cold rolling may be repeated as often as necessary. In addition, as indicated above, the alloys may be stabilized after the final cold roll by holding said alloys at a temperature of 200 to 450 F. for at least minutes and preferably 1 to 4 hours.

It is further preferred that all thermal treatments, including the preliminary heat treatment or homogeniza tion treatment, the hot rolling step, and subsequent interannealing of the hot rolled material, be followed by a slow, controlled cool down rate of 500 per hour or less to room temperature and preferably 50 F. per hour or less.

It should be noted that when the alloy contains indium, gallium or cadmium, the hot rolling temperature of these alloys should be maintained below 600 F. in order to prevent liquidation of micro-constituents, i.e., in order to prevent localized melting of the alloying ingredients.

The process of the present invention provides improved alloys in the cold rolled tempers. The greatest improvements are provided when the alloys are subjected to two (2) or more cold rolls with intermediate anneals and in particular when the alloys are in the cold rolled plus stabilized condition. When the alloys are in the cold worked temper they are characterized by a minimum yield strength of 45,000 p.s.i., with yield strengths generally on the order of 48,000 to 60,000 p.s.i., a minimum tensile strength of 55,000 p.s.i. and generally from 60,000 to 75,000 p.s.i. and a minimum elongation of 6% with elongations generally on the order of 8 to 10%,. After recovery, i.e., after the holding or stabilizing step, the alloys are characterized by a minimum yield strength of 35,000 psi. and generally from 37,000 to 55,000 p.s.i., a minimum tensile strength of 50,000 psi and tensile strength generally from 56,000 to 70,000 psi. and a minimum elongation of 12% with elongations generally from 15 to It is also quite surprising that the fully annealed properties of the alloys of the present invention are quite high as compared. to conventional aluminum-magnesium alloys, for example, the fully annealed properties of the alloys of the present invention are: yieldstrength, from 20,000 to 30,000 p.s.i., tensile strength, from 45,000 to 55,000 p.s.i., and elongation from 20 to 30%.

Theforegoing characteristics of the alloys of the present invention are particularly surprising and represent a considerable improvement over conventional alloys of this type.

In addition, the cold rolled properties, both before and after recovery, are characterized by good corrosion resistance and excellent stress corrosion resistance. These alloys, surprisingly, will not fail both in cold worked and stabilized tempers under prolonged exposure in the ambient temperature range, i.e., up to 180 'F.; whereas, all other alloys of this type will catastrophically fail under these conditions. The alloys of the present invention in the cold worked and stabilized tempers have been shown to hold up for one year and longer in'rigorous, natural environmental testing, with the test still proceeding without failure.

The melting and casting of the alloys is not particularly critical. The alloys may be melt and cast by any conventional method, such as, for example, the direct chill or tilt mold method. I

The alloys of the present inventionalso exhibit good physical properties as a cast product and will showa sig nificant strength advantage over conventional aluminummagnesium alloys. For this use, the alloys may be cast into final shape using conventional sand and permanent molding techniques.

In the preferred embodiment of the present invention, the following preferred amounts of materials are utilized: from 6 to 8% magnesium; from 0.1 to 0.2% chromium; from 0.05 to 0.60% indium; from 0.03 to 0.20% gallium; from 0.10 to 0.30% cadmium; from 1.0 to 0.30% germanium; from 0.10 to 0.40% lithium; from 0.01 to 0.10%, tellurium; from 0.15 to 0.50% hafnium; from 0.05 to 0.20% misch metal; from 0.02 to 1.0% thorium; from 0.01 to 0.05% boron. It is noted that misch metal is. a mixture of the rare earth metals, for example misch metal contains cerium, lanthanum, neodymium, didymium, etc. The preferred amount of manganese is from 0.10 to 0.40%. When manganese is present, one may also utilize zinc in an amount from 0.05 to 1.5%, with a preferred zinc content of from 0.10 to 0.50%. It should he noted that when copper is present, cobalt should also be present. The preferred amount of these materials is copper from 0.15 to 0.40% and cobalt from 0.15 to 0.60%.

In addition to the foregoing alloying additions, naturally the present invention contemplates the use of the normal impurity levels common to commercial grade aluminum. However, impurity ranges should preferably be maintained with the following limits: iron, up to 0.50%; silicon, up to 0.50%; titanium, up to 0.15%; beryllium, up to 0.02%; and others in total upto 0.2%. In fact, it may be desirable to add one or more of the foregoing materials in order to enhance a given property, for example, castability or to minimize staining during annealing. Beryllium is a preferred alloying addition in amounts from 0.0005 to 0.02%, and optimally from 0.001 to 0.005%. v

The present invention will be more readily understandable from a consideration of the following illustrative examples.

EXAMPLE I v Ingots were prepared of the alloys of the present invention in a conventional manner summarized as follows:

melting and alloying were carried out in an induction heating furnace. The melt was stirred after each alloying addition and just before fluxing, with the melt being degassed by gaseous chlorine fluxing at a rate of 3000 cc. per minute for 15 minutes. The melt temperature was maintained at 1350 to.l360 F. The charge .was then bottom poured using standard, direct chill casting techniques at an average casting speedof 3.5 to 4.0""per minute on a 3" x 6" mold section.

The alloys of the present invention were prepared in this manner and had the following composition:

Alloy A: Percent Magnesium c 7.41 Iron 0.30

Silicon 0.11

Copper 0.084 Titanium -1 a; 0.020

Beryllium 0.003 Chromium 0.16 Tellurium about 0.02

Alloy B: Percent Magnesium 7.1 Iron 0.27 Silicon 0.12 Copper 0.72 Titanium 0. l7 Beryllium 0.005 Chromium 0.14 Lithium 0.31 Alloy C:

Magnesium 7.6 Iron 0.27 Silicon 0.06 Copper 0.05 Titanium 0.016 Beryllium 0.002 Chromium 0.15 Germanium 0.21 Alloy D:

Magnesium 7.0 Iron 0.25 Silicon 0.16 Copper 0.075 Titanium 0.015 Beryllium 0.005 Chromium 0.15 Manganese 0.6 Alloy E:

Magnesium 7.0 Iron 0.25 Silicon 0.10 Copper 0.20 Titanium 0.015 Beryllium u 0.006 Chromium 0.14 Manganese 0.15 Zinc 0.35 Alloy F:

Magnesium 7.5 Iron 0.25 Silicon 0.095 Copper 0.064 Titanium 0.016 Beryllium 0.002 Chromium 0.15 Thorium 0.052 Alloy G:

Magnesium 7.05 Iron 0.28 Silicon 0.08 Copper 0.06 Titanium 0.005 Beryllium 0.002 Chromium 0.1 Boron 0.034

Alloy H:

Magnesium 7.15 Iron 0.22 Silicon 0.11 Copper a- 0.05 Titanium 0.013 Beryllium Trace Chromium 0.13 Hafniu-m 0.25

Alloy I:

. Magnesium 7.2 Iron 0.29 Silicon 0.12 Copper 0.26 Titanium 0.13 Beryllium 0.002 Chromium 0.15

Cobalt 0.56

6 Alloy I: Percent Magnesium 7.0 Iron 0.30 Silicon 0.14 Copper 0.074 Titanium -1 0.16 Beryllium 0.003 Chromium 0.15 Misch metal 0.10 Alloy K:

Magnesium 7.2 Iron 0.29 Silicon 0.11 Copper 0.069 Titanium 0.016 Beryllium 0.003 Chromium 0.16 Indium 0.090 Alloy L:

Magnesium 7.0 Iron 0.25 Silicon 0.07 Copper 0.05 Titanium 0.013 Beryllium 0.002 Chromium 0.15 Gallium 0.14 Alloy M:

Magnesium 7.0 Iron 0.25 Silicon 0.1 Copper 0.2 Titanium 0.015 Beryllium 0.005 Chromium 0.15 Cadmium 0.1

EXAMPLE II For comparative purposes, two alloys were prepared in the same manner as in Example I to have the following composition:

Comparative Alloy N: Percent Magnesium 7.2 Iron 0.05 Silicon 0.05 Copper 0.03 Titanium 0.004 Beryllium 0.001 Chromium 0.004

Comparative Alloy O: 7

Magnesium 7.0 Iron 0.255 Silicon 0.11 Copper 0.082 Titanium 0.015 Beryllium 0.005 Chromium 0.10

EXAMPLE III The alloy prepared in Examples 1 and H were homogenized at 950 to 975 F. for 16 hours at temperatures followed by slow cooling at a rate slower than 50 F. per hour to room temperature. The ingots were then hot rolled at 675 F. to 0.172 gage, except ingots K, L and M were rolled at 575 F. to 0.172" gage. This was followed by slow cooling at the above rate to room temperature, followed by cold rolling to 0.086" gage. The alloys were then interannealed at 800 F. for 4 hours followed by slow cooling to room temperature at the above rate followed by cold rolling to 0.060" gage. 'Ihe alloys were then cut up for testing with the following results:

TABLE I Yield Tensile Elongastrength, strength, tion, p.s.i. p.s.i. percent EXAMPLE IV The alloys treated in accordance with Example III in 0.060 gage were stabilized by heating to 300 F. and holding at that temperature for four hours. The alloys were then cut up for testing with the following results:

TABLE II Yield Tensile Elongap strength, strength, tion,

Alloy p.s.i. p.s.i. percent A. 39, 400 58, 800 16. 3 B 39, 500 55, 100 7. 5 O 36, 000 54, 100 17. 3 D 41, 800 60, 600 13. 2 E 40, 200 58, 900 14. 2 F 37, 200 56, 500 15. 4 G 36, 200 55, 200 I 18. 8 H 38, 900 58, 200 15. 8 I- 40, 600 59, 500 13. 8 J- 36, 800 55, 600 16. 8 K 38, 600 57,200 16 L 39, 700 58, 100 15. 3 M 39, 700 57, 100 14. 2 N 28, 000 47, 300 22. 7 0 37, 600 800 18 EXAMPLE V This example shows the surprising stress corrosion resistance of the alloys of the present invention. In this example various samples were subjected to environmental stress corrosion tests run in a rigorous atmosphere. The test consisted of exposing a pre-stressed sample to the elements on the beach at Daytona Beach, Fla, for a period of time until the sample showed failure by stress corrosion cracking. The sample was pro-stressed by bending in the shape of a letter U. Normally, the failure by stress corrosion cracking was first exhibited at the apex of the sample.

All alloys were tested, with each sample being tested in the following conditions: (1) five samples in the cold worked condition after the treatments of Example 111; (2) [five samples in the stabilized condition after the treatments of Example IV; and (3) five samples in the sensitized condition, a condition designed to exaggerate stress corrosion susceptibility. The sensitization treatment consisted of heating to 300 F., holding for 24 hours and cooling to room temperature. The results are shown in the following table:

TABLE III Alloy Condition Time to failure by stress corrosion cracking A Cold worked No failure after 15'months and still testing. A Stabilized Do. A.-. sensitized".-. No failure after 10 months and still testing. B Cold worked No failure after months and still testing.

E sensitized".-. No fa hrs after 10 months and still testing:

Alloy Condition Time to failure by stress corrosion cracking F Cold w0rked No failure after 15 months and still testing. I1- Stabilized, Do. 13 Sensitized. Do

.Cold worked No failure after 13 months and still testing.

Stabilized Do. sensitized'lfll; Nofailure after 10 months and still testing.

o. N0 failure after 10 months and still testing. Qoldworked No failure after 15 months and still testing. Stab1lized Do.

. sensitized..- No failure after 10 months and still testing. Cold worked- No failure after 1; months and still testing.

Stabilized. Do.

Sensitizei No failure after 10 months and still testing. Cold worked" N0 failure after 14 months and still testing. Stabilized"-.. Do.

sensitized..- No failure after 10 months and still testing. Cold worked. No failure after 13 months and still testing. Stabilized D0.

sensitized... No failure after 10 months and still testing. Cold worked" No failure after 15 months and still testing.

M Stabilized D0. M Sensitized.. N 0 failure after 10 months and still testing.

N Cold worked All samples failed from 111 to 185 days.

N Stabilized- All samples failed from 27 to 55 days. N. sensitized-.. All samples failed from 24 to 35 days. O Gold workerL; No failure after 12 months and still testing. 0-- Stabilized N 0 failure after 14 months and still testing. 0 sensitized, All samples failed from 100 to 300 days.

25 A further set of tests was run as above on alloy B "without chromium with the following results: in the cold worked condition, no failures after 15 months and still testing; in the stabilized condition, three out of live samples failed from 70 to 105 days; and in the sensitized condition,'all samples failed from 83 to 139 days.

A'further set of tests was run as above on alloy G without chromium with the following results: in the cold rworked andstabilized conditions, no failures after 13 months and still testing; in the sensitized condition, three out of five samples failed from '70 to 105 days.

A further'set of tests was run as above on alloy M Withoutchromium-with the following results: in the cold worked condition, three out of five samples failed from 98 to 2401 days; in the stabilized condition, three out of dive samples failed from to 60 days; and in the sensitized condition,-all samples failed from 26 to 42 days.

This invention may be embodied in other forms or carried out in other ways without departing from the. spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended tobe embraced therein.

,-What is claimed is:

1. A process for obtaining magnesiumcontaining aluminum base alloys having improved stress corrosion resistance which comprises:

(A) providing an alloy consisting essentially of from 5.5 to 10% magnesium, from 0.05 to 0.3% chromium, and the. balance essentially aluminum;

(8'). hot rolling said alloy at a temperature of from 450 to 950 F. to a gage of less than two inches; (G) cold rolling-saidialloy; l

(D) annealing said alloy at from 500 to l000 F.'-.f0f

'- at least '5. minutes and cooling said alloy to roomtemperature at a rate of less than 500 F. per hour; and

"(13) cold rolling said alloy. r

2. 'A- process for obtaining magnesium containing aluminum base alloyshaving improved stress corrosion resistance which comprises:

(Alproviding an aluminum base alloying consisting essentially of: from 5.5- to 10.0% magnesinm;from .1005 "to 0.3 chromium; a material-selected from 1 the group consisting of indium-from 0.002- to 0.80%,

zgallium from 0.01 to 0.50%, cadmium-from 0.03 to-0.50%, boron from 0.001 to 0.350%, thorium from 0.005 to 0.350%,misch metal from 0.005 to 0.30%hafnium from.0.05 to 0.7%,, tellurium" from 0.00510 0.30%., lithium from 0.01 to 0.80% manga:

nese.fromfi0.0S to-1-.0%, germaniumtrom 0.01 to 9 0.55%, and cobalt from 0.10 to 0.80% plus copper from 0.10 to 0.60%; balance essentially aluminum;

(B) hot rolling said alloy at a temperature of from 450 to 950 F. to a gage of less than two inches;

(C) cold rolling said alloy;

(D) annealing said alloy at from 500 to 1000 F. for at least minutes and cooling said alloy to room temperature at a rate of less than 500 F. per hour; and

(E) cold rolling said alloy.

3. A process according to claim 2 wherein prior to said hot rolling the alloy is homogenized at from 850 to 975 F. for from 5 to 30 hours.

4. A process according to claim 3 wherein after homogenization, the alloy is cooled to room temperature at a rate less than 500 F per hour.

5. A process according to claim 2 wherein after hot rolling the alloy is cooled to room temperature at a rate of less than 500 F. per hour.

6. A process according to claim 2 wherein after the final cold rolling step, the alloy is stabilized by holding at a temperature of from 200 to 450 F. for at least minutes.

7. A process according to claim 6 wherein said alloy is stabilized for from 1 to 4 hours.

8. A process according to claim 2 wherein said alloy is cooled to room temperature after annealing at a rate of less than 50 F. per hour.

9. A process according to claim 4 wherein both cooling rates are at a rate less than 50 F. per hour.

10. A process for obtaining magnesium containing aluminum base alloys having improved stress corrosion resistance which comprises:

(A) providing an alloy consisting essentially of: from 5.5 to 10% magnesium; from 0.05 to 0.3% chro- 3 mium; a material selected from the group consisting of indium from 0.002 to 0.80%, gallium from 0.01 to 0.50%, cadmium from 0.03 to 0.50%, boron from 0.001 to 0.350%, thorium from 0.005 to 0.035%, misch metal from 0.005 to 0.30%, hafnium from 0.05 to 0.7%, tellurium from 0.005 to 0.30%, lithium from 0.01 to 0.80%, manganese from 0.05 to 1.0%, germanium from 0.01 to 0.55%, and cobalt from 0.10 to 0.80% plus copper from 0.10 to 0.60%; and the balance essentially aluminum;

(B) homogenizing said alloy at from 850 to 975 F.

for from 5 to 30 hours;

(C) cooling said alloy to room temperature at a rate of less than 50 F. per hour;

(D) hot rolling said alloy at a temperature of from 450 to 950 F. to a gage of less than two inches;

(E) cooling said alloy to room temperature at a rate of less than 50 F. per hour;

(F) cold rolling said alloy;

(G) annealing said alloy at from 500 to 1000 F. for at least 5 minutes and cooling said alloy to room temperature at a rate of less than 500 F. per hour; and

(H) cold rolling said alloy.

11. A process according to claim 10 followed by stabilizing the alloy for 1 to 4 hours at from 200 to 450 F.

References Cited UNITED STATES PATENTS 3,232,796 2/1966 Anderson 148-11.5

HYLAND BIZOT, Primary Examiner W. W. STALLARD, Assistant Examiner I US. Cl. X.R. 

